1. Field of the Invention
This invention relates to gravity gradiometer systems and more specifically to a gravity gradiometer for use in navigation in moving vehicles and in other applications.
2. Description of the Prior Art
A gravity gradiometer instrument of the prior art has been described in a paper prepared by E. H. Metzger for the AIAA Guidance and Control Specialist Conference, Hollywood, Fla., Aug. 8-9, 1977 and entitled "Recent Gravity Gradiometer Developments", which is hereby incorporated by reference. The instrument employs rotating inertial sensors whose outputs are differentially summed to provide substantially simultaneous measurements of cross and in-line values of gravity gradients. The gravity gradient components may be expressed as a gravity gradient matrix, one such symbology in common use being: ##EQU1##
A cross gravity gradient is defined in the art as the rate of change of a given component of gravity at some point in space with translation of this point in space in a direction transverse to the direction of that component of gravity. The components G.sub.XY, G.sub.XZ, and the like are cross gradients. An in-line gravity gradient is defined as the rate of change of a given component of gravity at some point in space with translation of that point in space in the direction of the gravity component. The components G.sub.XX, G.sub.YY, and G.sub.ZZ are in-line gradients.
The aforementioned Metzger instrument is comprised of a continuously rotated cluster of four accelerometers to obtain continuous measurements pertaining to one cross component of the local gravity gradients and a combination of two in-line components. By mathematical analysis, both the in-line and cross terms may be computed. Data from the rotating accelerometer cluster is transferred by means of slip rings for utilization by an external computer. The external computer is also used to control cluster rotation speed, balance the accelerometer scale factors, and compute the gravity gradient components from the accelerometer outputs. However, this instrument is highly sensitive to variations in rotational speed. Because of this sensitivity, special features are required to reduce both rotation speed variations and their effects. These features are as follows:
1. An encoded disc and associated sensor to provide precise measurement of cluster angular position for use in controlling cluster rotation speed in a closed loop manner. PA1 2. Use of a four-accelerometer cluster, rather than a two-accelerometer cluster needed for a basic gradient measurement, to allow cancellation of the effects of residual speed variations and scale factor imbalances in the accelerometer outputs. PA1 3. Special motor-drive circuitry to induce a small high frequency fluctuation in cluster rotational speed. This allows detection and dynamic correction of imbalances among the scale factors of the four accelerometers. Such imbalances would otherwise cause imperfect cancellation of the effects of residual rotational speed variations. PA1 4. An external microprocessor to control rotor speed on the basis of the angular position data described above, and a firmware program using a second microprocessor to adjust the accelerometer scale factors on the basis of the effects of the forced rotor speed fluctuation introduced by the motor drive circuitry. PA1 1. Slip rings to transfer electrical power and gradient output data to and from the accelerometer cluster. PA1 2. The use of continuous demodulation and filtering to separate the in-line and cross gravity gradient components sensed by the instrument.
Additionally, the continuous rotation of the accelerometer cluster imposes two other requirements on the instruments: